In spite of nearly a decade of research on spin transport in graphene, there has been little improvement in important metrics such as the spin lifetime and spin diffusion length, and reported values remain far below those predicted by theory based on graphene's low atomic number and spin-orbit coupling. Understanding the extrinsic limiting factors and achieving the theoretically predicted values of these metrics is key for enabling the type of advanced, low-power, high performance spintronic devices envisioned beyond Moore's law. Scattering caused by tunnel barriers, which are essential for solving the conductivity mismatch problem for electrical spin injection from a ferromagnetic metal into a semiconductor, presents a major impediment to integration of graphene spintronic devices into modern electronic platforms. Uniform, pinhole/defect free tunnel barriers on graphene are not easily attained with the conventional methods that use oxides.
Recent studies using pinhole-free, high resistance tunnel barriers (e.g., monolayer BN and amorphous carbon) that minimally affect the graphene surface showed enhanced metrics for spin transport in graphene over previous work using conventional oxide tunnel barriers. Graphene itself exhibits many of the characteristics expected from a perfect tunnel barrier, including discrete monolayer thickness, imperviousness to interdiffusion, high uniformity, and high out-of-plane resistance, offering a new paradigm for tunnel barriers in magnetic tunnel junctions and spin injection into silicon. Moreover, in a graphene bilayer, by electrically decoupling the top layer from the bottom using chemical functionalization, a fluorographene/graphene homoepitaxial tunnel barrier structure showed the highest spin polarization achieved in graphene. Such functionalized homoepitaxial structures provide an elegant approach for realization of graphene-based spintronic devices, although spin-dependent operation was not achieved at room temperature, and fluorinated graphene has reduced stability compared to conventional dielectric materials.